This invention concerns support fittings for supporting a heat-resistant assembly to protect boiler tubes which is mounted on an array of tubes belonging to a heat exchanger (or boiler) of a heat recovery boiler in a waste incinerator or a thermal power plant. More specifically, it concerns the support fittings for supporting a heat-resistant assembly to protect boiler tubes. The heat-resistant assembly is used on the heating side of the tubes facing the incinerator to protect the array of boiler tubes which constitute the plant""s heat exchanger (i.e., boiler). This invention also concerns the array of boiler tubes on which the fittings are used.
FIGS. 4 and 5 show a combined structure of boiler tubes and heat-resistant assembly. 10 is an array of boiler tubes in a heat-recovery boiler. A number of cylindrical tubes which constitute boiler tubes 11 are arranged in parallel. Each two of the cylindrical tubes are fixed in place by the flat rib 12. In the center of each of the ribs 12 is welded a support fitting 100, which is oriented vertically.
Support fittings 100 consist of a parallelogram with a vertical surface 100a, which is welded to flat rib 12, and an oblique upper surface 100b, which will engage in a heat-resistant catch 19. Generally, the vertical surface 100a is first placed in contact with the flat rib 12, and then side foot portion 100c is welded by hand. 16 is the heat-resistant block. The tube assembly 10, consisting of the boiler tubes 11 and the flat ribs 12, must be protected from the heat and corrosive atmosphere of the exhaust gases from combustion. As can be seen in FIG. 4, tube assembly 10 is enclosed in such a way that a 180xc2x0 portion on the bottom of each tube is entirely covered. Each two lead tubes, paired in the axial direction, are protected by a block whose cross section resembles two semicircular tubes joined by a flat rib surface 16b. This block extends for a given length along the longitudinal axis of the tubes. The block surrounds boiler tube assembly 10 and is fitted close to but not directly against it, with a specified gap left between the block and the tube assembly.
19 is a heat-resistant catch by which the heat-resistant block 16 is attached through the support fitting 100 to boiler tube assembly 10 in such a way as to be integral with the assembly. It is a rectangular projection from the surface of flat rib 16b in the heat-resistant block 16. To insure that it has sufficient strength in the axial direction, the heat-resistant catch 19 should be no more than one third of the length of heat-resistant block 16. The catch can be attached to the heat-resistant block in not only one place, but more than two places.
The heat-resistant block 16 and the heat-resistant catch 19 are formed by molding a material like SiC which has relatively good thermal conductivity.
A thin layer of mortar 14 is packed on the inner side of the heat-resistant block 16 and the catch 19 to enhance the cooling effect of the block 16.
The boiler tube assembly 10 and the heat-resistant block 16 configured as described can be securely joined by means of support fitting 100 and heat-resistant catch 19. According to the prior art, the support fitting 100 was hand-welded to the flat rib 12 which connects two boiler tubes 11.
Because there are normally two semicircular boiler tube sections 11 in the location where the two facing surfaces must be hand-welded, the space into which the welding jig must be inserted is very small. In other words, the surfaces of the two boiler tube sections 11 interfere with the welding, making the welding task difficult and extremely time-consuming.
We therefore investigated the possibility of employing the comparatively simple procedure of stud welding instead of the hand welding. However, with both the arc and percussion stud welding, there were problems due to the non-rectangle shape of the support fitting 100 which made stud welding very difficult to perform.
Furthermore, the support fitting 100 does not have a round cross section like the stud bolt used in the prior art, but is tall and thin. It is difficult to achieve either the pressure or the temperature needed to weld it properly. When the long narrow fittings are to be stud-welded, they are frequently arc stud welded using a ferrule. To maintain the temperature for two-surface arc welding, a heat-resistant porcelain ferrule must be put on the end of the stud, and the welding must be executed while the periphery of the stud is covered by the ferrule.
The principle of arc stud welding using a ferrule can be explained simply with reference to FIG. 6. Ferrule 20 is placed on the end of stud 110. The end of stud 110 is placed in direct contact with a base metal. When the welder pulls the trigger of the welding electrode, a current flows between the stud 110 and the base metal 112.
The lifting mechanism 111 of the welding electrode automatically pulls up the stud 110. Inside the ferrule 20, an arc 113 is generated between the stud 110 and the base metal 112 as indicated by the arrows. The arc 113 is maintained for a period determined by a timer. The stud 110 and the base metal 112 fuse, and after a given period of time, the stud 110 is pressed against the base metal 112 and the current is cut off.
With this technology, then, a deoxidizing conductive material 115 as flux which is attached to the end of stud 110 by various methods acts on the metals in such a way as to result in a welded portion 114. The ferrule 20 mainly serves as a mold for the molten metals. When the welding is completed, it is removed as needed by a means such as breaking it.
However, when the support fitting 100 is welded by arc stud welding using a ferrule, the ferrule 20 cannot completely seal the welding surfaces of the materials as shown in FIG. 5(B), so it cannot serve as a mold.
Since support fitting 100 is to engage with the heat-resistant catch 19, it must have a stopper on its upper surface. This is why the surface which is to engage with the catch 19 slants upward. When the ferrule 20 is inserted onto the support fitting, as can be seen in FIG. 5(B), the upper surface of the fitting 100 is not perpendicular to the welding surface of the base metal (i.e., it is not horizontal). The ferrule 20, will be also oriented obliquely, so that its lower portion is not flush against the welding surface, making it difficult to maintain a uniform temperature.
Because the fitting 100 is long and narrow, it will be extremely difficult to insure that its contact with the base metal at the welding surface is uniform. If the arc is started from the lower end or the upper end where a considerable contact pressure is provided, a lopsided weld may result.
In view of the problems described above, the objective of this invention is to provide a support fitting for a heat-resistant block to protect boiler tubes which can be easily and reliably stud-welded without losing any of its function as a support fitting.
This invention concerns a support fitting for the heat-resistant block to protect boiler tubes, which protrudes upward at a right angle from the surface of the rib between two boiler tubes. The support fitting is welded on the rib and it has a catch to engage with the heat-resistant block on its end.
The support fitting according to this invention is distinguished by the fact that the welding surface of the support fitting to the rib is shaped narrower, and by the fact that a single globule of a deoxidizing conductive material used as flux is attached to the narrowed welding surface.
With this embodiment of the invention, even though the shape of the support fitting is long and narrow, the fact that its welding surface is narrowed makes it easier to achieve uniform contact with the base metal and increases the contact pressure on the welding surface. And because the deoxidizing conductive material used as flux is stuck to the constricted welding surface in a single globule, the arc can be started from the globule so that there is no possibility of a lopsided weld.
Because the constricted area is fused in its current state during arc stud welding, there is no possibility that this portion will be undercut. In other words, the surface should be constricted so that it cannot be undercut.
Because the welding surface is constricted, the fused portion will not extend very much beyond the periphery of the support fitting. Thus the ferrule placed on that periphery will not become trapped in the molten metal.
This invention, then, makes it possible to use arc stud welding using a ferrule easily and reliably without sacrificing the function of the support fitting.
In another preferred embodiment of this invention, if the support fitting is a vertical piece which extends a fixed distance perpendicular from the rib, the first upper surface of the perpendicular support fitting, which supports the ferrule, is kept horizontal, and the second upper surface of the perpendicular support fitting, which engages with the heat-resistant block on its end, is angled slightly upwards. The first upper surface is provided with a horizontal portion which can tightly engage with the ferrule for arc stud welding.
With this configuration, when the support fitting 100 is arc stud welded to the base metal, the first upper surface of the support fitting which supports ferrule 20 makes a right angle (i.e., it is horizontal) with respect to the welding surface of the base metal. This results in the ferrule also supported parallel to the welding surface of the base metal. In other words, the entire surface of the ferrule is flush against the surface of flat rib 12 (the base metal) without any gap at its lower end. This facilitates maintaining a uniform temperature. The ferrule completely seals the welding surface of the base metal, so it can fulfill its role as a mold.
In yet another preferred embodiment of this invention, the support fitting for the heat-resistant block to protect boiler tubes has a vertical piece which extends a fixed distance perpendicular from the rib, and a catch to engage with the heat-resistant block, which extends upward from the end of the vertical piece.
The invention is distinguished by the fact that the welding surface of the support fitting to the rib is shaped narrower, and a single globule of a deoxidizing conductive material used as flux is attached to the narrowed welding surface, and further by the fact that the vertical piece and the catch engage with each other in double groove style.
With this embodiment, in addition to the effects mentioned above, the support fitting is bifurcated to form a groove in the vertical piece which engages with a similar groove in the catch. This allows even support fittings with complex shapes to be manufactured easily by combining stainless steel plates, and it allows extremely heavy heat-resistant blocks to be locked securely into place.
In yet another preferred embodiment of this invention, the support fitting for the heat-resistant block to protect boiler tubes has a vertical piece which extends a fixed distance perpendicular from the rib, and a catch to engage with the heat-resistant block, which extends upward from the end of the vertical piece.
The invention is distinguished by the fact that the welding surface of the support fitting to the rib is shaped narrower, and a single globule of a deoxidizing conductive material used as flux is attached to the narrowed welding surface, and further by the fact that the vertical piece and the catch are cast from a heat-resistant metal comprising no more than 0.1% C by weight; no more than 2% Si by weight; no more than 2% Mn by weight; no more than 0.045% P by weight; no more than 0.040% S by weight; from 19.00 to 22.00% Ni by weight; and from 23.00 to 27.00% Cr by weight.
With this embodiment, a cast metal can be used which is produced by minimizing the proportion of C in the existing cast stainless steel SCS18. This will maximize the metal""s resistance to corrosion and its welding capability, and enable it to be formed into complex shapes.